Myeloperoxidase (MPO) is a specific enzyme of polymorphonuclear leucocytes (also known as neutrophils), which are white blood cells specialized in the fight against micro-organisms by phagocytosis.
Pathogens are destroyed inside the neutrophils by proteinases and myeloperoxidase, this latter enzyme being specifically responsible for the production of a potent oxidant agent, hypochlorous acid (HOCl). This HOCl (bleach) allows the destruction of the bacterial polysaccharidic capsules that withstand proteinases. MPO thus plays a key role in the host defense against infection.
During their fight against micro-organisms, dying neutrophils release myeloperoxidase in the surrounding liquids and tissues. When the activation of the neutrophils is excessive and becomes uncontrolled (as in acute inflammation pathologies), the release of myeloperoxidase is important and high concentrations of this enzyme are reached in biological media or samples (plasma, tissues, ascite fluids, broncho-alveolar fluids, pleural fluid, lymph, urine, saliva, uterine irrigation liquids . . . ), leading to an increased risk of cytotoxicity (by myeloperoxidase capture into cells or binding on cell surface, with in situ production of oxidants).
Until now, in equine medicine, myeloperoxidase was measured by the detection of a peroxidase activity. However, said detection according to hitherto developed techniques is not specific for equine myeloperoxidase (a general peroxidase activity is detected), is tedious and not applicable to complex biological media and samples (such as plasma) due to the presence of proteins (albumin, lipoproteins, . . . ) and reducing agents that interfere with the enzymatic measurement.
No efficient technique has been developed hitherto for the measurement of the total concentration of equine myeloperoxidase in complex biological fluids (both cellular and acellular) and in tissues.
The presence of myeloperoxidase in alveoli and tissues is presently estimated by the measurement of a non-specific peroxidase activity after extraction.
There is an ever increasing demand for easy and improved diagnostic tests to measure MPO activity and concentrations, especially in horses. The design of a rapid and sound assay for equine myeloperoxidase measurement is highly wanted for the diagnosis of equine diseases and/or pathologies (such as for instance colics with a high mortality in Equidae, sepsis, acute lung injury, acute inflammation, . . . ). The choice between a clinical treatment, a surgical intervention (costly for the veterinary surgeon and for the breeder) or, at the worst, euthanasia of the animal will be easier to make and the decision taken will be better funded when good, rapid and reliable assays exist to diagnose excessive neutrophil activation and/or invasion.
Myeloperoxidase is known to be a specific marker for excessive neutrophil activation and/or invasion, in humans as well as other mammals such as horses. In horses, intestinal tissue scores correlated for instance positively with tissue MPO activity in adjacent specimens (Mc Connino et al., 1999, Am J Vet Res 60: 807-813). It has been established that the physiological values of plasma myeloperoxidase in healthy horses are significantly exceeded in several acute abdominal pathologies, in horses with large intestine strangulation and in horses which will not survive (Deby-Dupont et al, 1998, Vet Immunol Immunopathol. 66: 257-271; Grülke et al., 1999, Can J Vet Res. 63:142-7; Grülke, 2002, doctoral thesis, ISBN 2-930212-57-8). In humans, the plasma concentration of myeloperoxidase is currently taken as marker of neutrophil activation such as in cardiovascular diseases (Zhang et al., 2001, JAMA 286: 3126-2142).